In the specification of the present invention, the term "heat stability" means resistance to thermal decomposition such as discoloration and deterioration, and the term "heat resistance" means resistance of a molded resin to thermal deformation.
Nonflammable resin compositions composed of a low-molecular-weight styrene resin and a low-polymerization-degree vinyl chloride resin are being used in increasing amounts as materials for housings of office automation (OA) apparatuses owing to their superior mechanical properties and low cost.
These nonflammable resin compositions, however, are difficult to mold because of their low heat stability and are liable to cause burning or mold flashing of molded products. This problem is considered to result from the vinyl chloride resin contained in the nonflammable resin composition increasing in stickiness during injection molding, leading to sticking to metallic parts such as a screw, cylinder, mold, or the like to cause partial stagnation of the resin and heat generation by shearing, thereby resulting in thermal decomposition of the resin.
For offsetting the above disadvantage, a large amount of a stabilizer or a lubricant is generally added to the nonflammable resin composition to improve the heat stability.
This method, however, is unsatisfactory because of the high cost of the starting materials and poor quality of the product such as low heat distortion temperature, namely, low heat resistance.
In view of such circumstances, the inventors of the present invention made a comprehensive investigation and considered the fact that the nonflammable resin composition is not a high-molecular styrene resin generally used as an injection-molding resin, but is a specific injection molding resin composition composed of a low-molecular styrene resin blended with a low-polymerization-degree vinyl chloride resin. As the result of their investigation, the inventors have found that the heat stability of the nonflammable resin composition can be improved by the addition of a specific copolymer as a modifier without impairing heat resistance and without adding a stabilizer or a lubricant as disclosed in JP-A-2-305840, JP-A-2-11644, JP-A-1-236252, JP-A-1-221447, JP-A-63-69854, and JP-A-63-63736 ("JP-A means unexamined Japanese patent publication).